Phase modulator and clipper



June 7, 1960 w. J. BROWN PHASE! MODULATOR AND CLIPPER 7 Sheets-Sheet 1Filed June 22, 1956 o a 3 I.

IMO 2000 I O0!) LATINO FR EOUEHGY FIG 3000 CPS IODULATIHB FREQUENCY FIGZINVENTOR. WALTER J BROWN June 7, 1960 w. J. BROWN PHASE MODULATOR ANDCLIPPER 7 Sheets-Sheet 2 Filed June 22, 1956 PHAS E HODULATED R F OUTPUTFIG 3 INVENTOR. WALTER J BROWN FIG 4 June 7, 1960 w. J. BROWN 2,940,055

PHASE MODULATOR AND CLIPPER Filed June 22, 1956 7 Sheets-Sheet 3 Kl F .TH 1 L.....- U291 i i l l I I I I IQ l8 l' 6 l9 INVENTOR. b 6 WALTER JBROWN 4M4 K oal-7" June 7, 1960 w. J. BROWN 2,940,055

PHASE MODULATOR AND CLIPPER Filed June 22, 1956 7 Sheets-Sheet 4 FIG 8INVENTOR WALTER J BROWN June 7, 1960 J w. J. BROWN 2,940,055

PHASE MODULATOR AND CLIPPER Filed June 22, 1956 7 S heets-Sheet 5 ALL 1l 1..-..- l I I I 1 l g I I 44 46 l U .ZZIff i 42 45 I I /l v I 43 47 4g4? H l- 50 5| 52 FIG 9 INVENTOR WALTER J BROWN BY June 7, 1960 W. J.BROWN PHASE. MODULATOR AND CLIPPER Filed June 22, 1956 7 Sheets-Sheet 6INVENTOR WALTER J BROWN J1me 1960 w. J. BROWN PHASE MODULATOR ANDCLIPPER 7 Sheets-Sheet 7 Filed June 22, 1956 PS1: El

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INVENTOR. WALTER J BROWN United States Patent PHASE MonULAToR ANDCLIPPER Wa'IferILBroWn, 7i Gufly Road, Stamford, caim. Filed June 22",1956, so. No. 593,183 12 Claiins. Cl. 332-29 This invention pertains ingeneral to phase modulators and more particularly to a phase modulatingsystem having a characteristic such that a given audio frequency'modulating" current produces the same phase deviation, regardlessof themodulation frequency.- Reference may be had to the phase shiftingcircuits of my U.S. Patents Numbers 25243759, 2,524,760, 2,524,761,2,524,762. This invention may he used in someof thecircuits described inmy co-pending application Serial Number 575,133, datect March 30 1956-,entitled Stabilized Phase rlo'dnlato'n g object of my invention is toprovide a phase moan later: in which the audio source supplies the samecurrent at all frequencies" to the phase shifting network,

Another" object ofmy invention is to prov-idea phase modulator us'eah'lein combination with a fixed carrier frequency source to produce aconstantphas'e deviation with constant modulating current, regardless ofthe meanlation frequency.

Another object of my invention is to provide a phase modulator whichproduces a frequency deviation directly proportional to the modulationfrequency when supplied with a constant modulating current.

Another object of my invention is to provide a phase modulator"incorporating a clipper circuit without vacuum" tubesto' limit themaximum frequency deviation,

Another object of my invention is to provide a phase modulatorminimizing the elfect ofvariatio'ns' in the D.Cl modulation supplyvoltage;

Another object of my invention is to provide awid e' angle phasemodulator having a minimum of distortion.

Anotherob'j'ect' of my invention is to provide a phase modulatorrequiring no microphone transformer in the audio circuit,no'pre-amplifier' tubes, no phase modulator tube's' a'nd, when' thecircuit is'used in a frequencymo'dulatioh' system, no" first multipliertube. v

Other objects and a fullerunderstandi'ng' of my inven-' ti'on may be hadbyrefe'rring to the following description and claims taken inconjunction with the accompanying drawings, in" which:

Figure l is a graph showing the radio frequency devia'tion' required atvarious audio modulating frequencies in a frequency-modulated mobileradio transmitter.

Figure 2 is another graph showing the corresponding phase deviationcharacteristics which are required in' circuits'elmploying a phasemodulator to produce frequency modulation; p A v Figure" 3 is asimplified drawing or a" phase modulator circuit employing one of thecorrective means of my" inventiozi.

Fi"g11 r"4 i'$'a voltage vector diagram relating to the circuit ofFiguie3".

Figure 5 is n'zic'rophone cirouitw Figureio isa drawing of similarcircuit employing! a double buttb'n- 'rnicrophone.-

Figure 7- is-a drawing of a clipper device for limitingfrequencydeviation in the phase modulator.

another drawing of a" phase modulator" Patented June I, 1960 Figure 8'is a drawing showing another clipper arrangelfilat.

Figure 9 is adrawing showing still another arrangeni'e'iit Figure 10 isadrawing' showing a clip-per circuit conneclted directly in themicrophone circuit.

Figure 11 is" a arms-g showing an arrangement for limiting frequencydeviation including a double button microphone.

Figure 12 is a diawmg showing a complete phase modulator in accordancewith my invention.

Figure 13 is a voltage vector diagram relating to the circuit of Figure12.

The circuitsdesoribed in this invention are useable in a phase modulatorsystem employing a carrier source of fixed phase and frequency, such asa crystal controlled oscillator, in combin ationwith a phase shiftingnetwork which includes a saturable reactor having a radio frequencywinding connected in the phase shifting network and an audio frequencycontrol winding connected to an audio source of modulation. The audiosource of modulation may be of the types shown in Figures 3, 5, 6, 7, 8,

9', 10, ll and 12. The audio frequency component of the microphonecurrent passes thru the audio winding of the saturable reactor an'dthisproduces a cyclical change in impedance of the RF winding andconsequently a phase rri'tat all frequencies to the audio frequencycontrol windingof the saturable' reactor the maximum phase deviationwill be constant at all modulation frequencies and the maximum frequencydeviation will increase in direct proportion to the modulating frequencyas shown in Figures 1 and 2. Figure 1 shows the basic frequencycharacteristic which is required under present regulations of theFederal Communication Commission for an FM mobile communication system.

The frequency deviation of the RF output of the transmitter, for a givenaudio modulating amplitude, rises in direct proportion to the modulatingfrequency from 300 c.p'.s. to 3000 c.p.s. as shown by the sloping solidline whioh shows the frequency characteristic for modulation; However,itis also required that the maximum frequency deviation be limited to:15 kc. as shown by the horizontal dotted line, in order to preventinterference with adjacent communication channels.

In an FM transmitter of the phase modulated type, an oscillator sourceof fixed phase and frequency, such as a crystal oscillator, is used andits output is then phase modulated and the frequency of the phasemodulator is then multiplied sufficiently to give the required RFfrequency deviation. The RF frequency deviation produced by a givenphase deviation is known to be proportional to the modulating frequency.Accordingly, the phase deviation required to produce the frequencycharacteris tic of Fig. l is as shown in Fig. 2. The phase deviationshould be constant at all modulating frequencies, as

shown by the horizontal solid line which corresponds to" Insuch phasemodulated transmitters in the prior art itwas necessary to employavacuum tube as the variable impedance element in the phase modulatornetwork together with a-pre amplifier tube to augment themodulat ingsignal obtained-from the microphone; it was alsofre quently necessary toinclude a predistorter or preemphasizing networkiirorder that-the highaudio'frequencies might be amplified to a greater extent than the lowaudio frequencies, so as to give the desired rising char acteristic offrequency deviation shown in Fig. 1. Aecording to the present invention,the needs for amplification. of the audio modulating signal, and foramplifying the high audio frequencies toa' greater extent than the low 4audio frequencies, are eliminated by using a saturable core reactor asthe variable impedance element in the phase modulator network. Thesaturable reactor preferably comprises a radio-frequency magnetic coreportion of ferrite, powdered iron or powdered iron oxide carrying aradio frequency winding which is connected in the RF network of thephase modulatonand an -andiofrequerrcy V magnetic core portion oflaminated iro'n or iro n alloy carrying an'audio frequency winding whichis connected in the audio network of the phase modulator. j The magneticcoreportions and the windings are so arranged that a modulating currentin the audio frequency windingwill vary the saturation andhence thepermeability of the magnetic core portion within the RF winding, whilstthe inductive coupling between the RF and audio frequency windings isminimized or neutralized by dividing either f the RF or the audiofrequency winding into two sections having relatively opposed magneticcouplings. A permanent magnet or DC. winding may also .be included forthe purpose of magnetically biassing the" RF core portion to its mostfavorable condition. j

' Referring now to Figure. 3, microphone 10 is connected 1 ,25 a controlwinding 2470f a saturable'reactor 25,.preferably 7 4 modulatingfrequency are shown in Figures 5 and'6 which show alternative audiofrequency circuitsrto that shown in Figure 3.

In Figure 5 the carbon microphone l iand battery 15 are connected to thecontrol winding 16 of a saturable reactor 20 through condenser 18 andpreferably through resistor'17 which is shown in dotted The DC.

component of the microphone current does not appear in f the controlwinding 16 as'i-t is lay-passed thru resistor 19.

The winding 16 has relatively few turns .of' wire, so that its reactanceat audio frequency is low compared with the effective resistance of themicrophonecircuit. The saturable reactor 20 has an audio frequency coreportion 20A and a radio frequency core portion 20B as shown in dottedlines. The permanent magnet 20C is used to bias the core of thesaturable reactor magnetically, so that its saturating flux is increasedor decreased according to whether the instantaneous microphone currentis increasedordecreased. The winding 8A, 8B is a radio thru resistor 13and battery 11 to the control or audio coupled inopposite senses to thetwo .por tions 8A and 8B of the saturable reactor. The saturable reactor9 has a' laminated audio-frequency core portion 9A'and afradio-frequencycoreportion9B comprisingai ring-shaped member of ferrite, powdered ironor powdered} iron oxide. The RF'winding portions 8A and 8B are.windingrlz of a saturable reactor-9 which is magnetically seriallyconnected and they are, in turn connected in V series with a cond'enser6 thru an output terminal 7 across the terminals 3, 5 of acondenser 4.The condenser 4 is energizedby a resistor .2 which is connectedin'series' with it across the terminals 0, 3 of a constant frequency RFsource 1, such as a crystal-controlled oscillator. The

' terminals 0, 3, comprise the input terminals of the RFphase-modulating network, and the terminals 0, 7, comprise its outputterminals. V j

Figure 4 is a vector diagram showing'the relationships 2 of base linevoltage A'B" taken across terminals '3 and 5 to the input voltage O-Awhich represents the voltage frequency winding which is connected as thevariable inductance in a phase shifting netWQI such as is shown inFigure 3 or in one of the networks described in US. Patents 2,524,759,2,524,760,- 2,524,761 or'2,524,76 2.

;Figure 6 shows another arrangement employing a double gbuttoncarbon'microphone 21 connected to the through resistors 2-3and 26, A D.Csource such as battery 22 is connected between the microphone anda-center tap of the winding 24 to provide microphone current; theaverage D.C.xcuirents in the two halves of the winding arebalancedoutinthis arrangement, so that thebatterycui'rerit does notproduce any appreciableunidirectional magnetomotive. force in. thecorerThe saturable reactor an audio frequencylcoreportion 25A and an coreportion. 25B,Land abiasing'imagnet' 256 similar to the magnet ii-{Figure5 maybe eniployed to provide the necessary steady unidirectional Again,the winding24 is provided withrelatively few turns so that its audiofrequency reactance is small compared with the effective resistance ofthe microphone circuit. The radio frequency winding 8A, 8B 'is connectedin a phase shift ing network as previously described.

'It is necessary in many practical systems to provide a clipperarrangement for the purpose of limiting'the across RF oscillator lwhichhas constant phase and fre- V quency. PA is the voltage acrosstheinductive elements 8A and 813 between terminals 7 and,3, and PB is thevoltage across the capacitive element 6 between the terminals 7 and 5.When the signal current thruthe con-' trolwinding 12 is increased, theinductance of the wind-' ings 8A and 8B is decreased due to coresaturation, thus resulting in a reduction of the voltage PA" in relationto the voltage P'B; if the saturable reactor is designed so that its'Qor phase angle remains approximately constant whileits inductance isvaried by'saturation, the angle A'PB will remain constant while theratio of the amplitudes P'A' and PB' is varied and accordingly the pointP will move around the arc of a circle spanning the baseline AB 'asshown in dotted lines. By suitable said arcuate locus; and'the'outputvoltage O'P' may then be varied in phase While remainingsubstantially'constant'in amplitude. Y

Referring back to Figure 5, I have found that a constant current .in thecontrol winding" of the saturable re actor can be produced; (regardlessof-its' frequency) by.

.making the inductance of the'control winding low in're- 'lation to theresistancebf :the microphone circuit; This result may be achieved byadding resistance 13, as shown in dotted lines, in series with themicrophone circuit, 'or

"alternatively by winding the coil 12 with a small number a of turns ofwire. "Other arrangements for ensuring that the saturable reactorcontrol current is independent of-the" design the point 0 maybe locatednear to the center off.

' Value; the clipper windingfilhasianenter tapped.

,D.C. source such as battery 34 in combination with a frequencydeviation and preventing interference Withfad jacent channels, inaccordance with regulations, of the Federal Communications Commission;The'clippfir must operate at a predetermined radio frequency deviation,and

musttherefore operate at ,a'phase deviation which is. inverselyproportional to the audio modulating frequency, 1

as shown 'by'the dotted lines in Figures 1 and 2. This resultmay beaccomplished as follows. Referring to Figure '7, microphone-27 isconnected to control winding 29 thru resistor'30 and battery 28. Controlwinding 29 is a winding of a s'aturable reactor 35 havingan audio A.C.short circuit which comes into effect only when the voltage induced inthe winding exceeds ,a predetermined opposing D.C. reference voltagesprovided-;by-,-each1half ofithe D.C. source, so-tha't, the rectificrsonly conduct when the voltage induced in winding jtexceedsrthe.

; voltage; of one half of the DC; source.- The 'saturable reactor isagain provided with an R.'F. wind ing' -8A, 8B- which is connected in aphase=shifting networkas-prevh '5 ously described. A variation of thisarrangement is shown inFigure 8 in which control winding 36 is coupledto a center-tapped clipper winding 37-38, the end ter minals of which inturn are connected through back-toback rectifiers 40 and 41 and battery39 to the center tap of the clipper Alternatively, the DC. referencevoltage may be omitted by using a rectifier such as a multiple plateselenium rectifier which does not conduct until an appreciable positivevoltage has been reached. Such an arrangement is shown in Figure 9 inwhich microphone 42 is connected to control winding 44, resistor 45 andbattery 43. Clipper winding 46 is connected to a pair of back-to backseriesconnected rectifier groups consisting of rectifiers 47, 48, and 49in one group and rectifiers 50, 51 and '52 in a second group.

The clipping effect can be accomplished without use of a separateclipper winding by the arrangement shown in Figure in which microphone53 is connected thru battery 53A and resistor 54 to the control winding55 of the saturable reactor. Rectifiers 56 and 57 are placed in oppositedirections across the control Winding 55. Bias or reference Voltage maybe included as indicated by the dotted lines 56A, 57A connecting eachrectifier to one side of the control winding.

Preferably the total resistance of the microphone 53 and the resistor 54is high compared with the reactance of the winding 55 over the requiredrange of audio frequencies and, accordingly, the alternating componentof the current thru the winding 55 is substantially independent of themodulation frequency. Also, the ohmic resistance of the winding 55 ispreferably very low compared with the total resistance of microphone 53'and resistor 54, so that the DC. voltage drop across winding 55 isnegligible. However, the AC. voltage drop across winding 55 isproportional to the A.C. component of the audio current multiplied byits frequency, and it is therefore proportional to the RF phasedeviation multiplied by the audio modulation frequency, and is thereforeproportional in turn to the frequency deviation in the RF output of thephase modulator. The rectifiers 56 and 57 are selected to have a lowimpedance, relative to the audio-frequency reactance of winding 55, whenthere is sufiicient voltage across said winding to cause them to conductand, accordingly, the rectifiers will clip olf and bypass any componentsof the modulating current which would otherwise cause excessive radiofrequency deviation. The bias or reference voltage sources 56A and 57Amay take the form of a battery or other D.C. source with tappingsthereon, or alternatively it may simply comprise the threshold voltageof the rectifiers 56 and 57, below which voltage they are substantiallynon-conducting. If the bias or reference voltages 56A and 57A aresupplied from actual voltage sources, one of said voltages may be lowerthan the other, or may be entirely omitted, to compensate for the DC.voltage drop which inevitably exists due to the microphone current inthe winding 55.

Another arrangement is shown in Figure 11 using a double button carbonmicrophone 58 in which the microphone 58 is connected thru resistors 60and 61 to opposite ends of the control winding 65. Battery 59 isconnected between the microphone and a center tap 65A of the controlwinding 65. Rectifiers 63 and 64 are connected back to back acrosscontrol winding 65 and are biased by battery voltage supplied by thepotential divider 62. Potential divider 62 may be provided with avariable tap 62A to control the maximum radio frequency deviation bymeans of the clipper circuit.

Figure 12 shows a complete phase modulator. The microphone circuit issimilar to the arrangement of Figure ll. Double button carbon microphone76 is connected thru resistors 73 and 79 to the center tapped controlwindings 85 and 37 of saturable reactor 83. Rectifiers 81 and 82 areconnected back to back and biased thru a tap 80A on potential divider 80connected 6 across battery 77 which maybe any sourceof DKC. voltage.Rectifiers 81 and 82 provide a clipper'efiect limiting the maximum radiofrequency deviation as'discuss'ed above. The audio frequency voltageacross windings and 87 is proportional to the modulation frequency sothat rectifiers 81 and 82 conduct and limit thepeakvoltage to a valuewhich is determined by the biasing or reference voltage from resistor 80and which corresponds to a definite radio frequency deviation. Theresistors 78 and 79 ensure that the audio frequency current in the controlwindings is substantially independent'of the audio frequency. Thesaturable reactor 83 may have a laminated core structure 83A as shown inthe dotted lines, carrying the control windings for the audio frequencypath, and may have a pot core of powdered iron or ferrite, carrying thewinding 89 in the radio frequency path. The polarity is such that theaudio frequency control windings 85 and 87 assist each other in drivingan audio frequency flux from end to 'end of the pot core '90; on theother hand, the radiofrequency current in winding 89 produces an RF fluxwhich circulates between the central core and the outer shell of the potwithout traversing the laminated section of the core. The permanentmagnet 84 is arranged in shunt with the magnetic audio circuit includingthe pot core and is adjusted to provide the optimum magnetic biassingflux in the audio frequency'magnetic circuit and the pot core. Thedouble button carbon microphone 76 produces a roughly equal and oppositeD.C. magnetization with respect to the laminated core structure of the"saturable reactor thru windings 85 and 87. An RF carrier source voltage91 is supplied to a resonant tank 'cir cult comprising inductance andcondenser 94.

The RF saturable reactor winding 89 is connected in series withcondenser 102, thru the output terminal (P) across the tappings A; and Bon theinductan'ce 95 and the voltage across these elements may berepresented by the vector triangle AP'B' in Fig. 13, in which the pointP may be moved around its arcuate' locus, shown as a dotted circle, byvarying the inductance of the winding 89.

Resistor 92 and condenser 93 are connected in series, thru a secondoutput terminal 111 (0), between points 98 (X) and 1&1 (Y), across theinductance 95, and their voltages may be represented by the vectortriangle X'O'Y' in Fig. 13, in which 0 is near the center of the arcuatelocus of P. Accordingly, an output voltage 0"? may be taken from thephase modulator which can be varied in phase over a wide angle whileremainingsu b stantially constant in amplitude.

in order to correct the position of point P'so that it assumes asymmetrical position close to P under quiescent conditions, rectifiers103, 104 are provided in conjunction with condensers H57, 168 andresistors 105, 106, for the purpose of developing D.C. voltages acrosscondensers 167, 168 which are generally proportional to the AC. voltagesX'P and PY in the vector diagram of Fig. 13. These D.C. voltages areapplied to opposite ends of the saturable reactor correcting windings 86and 88 with such polarity that any difference between said D.C. voltagesalters the core saturation so as to move the point P towards itssymmetrical position P RF chokes 112 and 113 are provided to minimizehigh frequency losses, and furthermore the condensers 107, 108 andresistors 1&5, 106 and the windings 86, 88 are so chosen as to provide arelatively long time constant in comparison with the period of thelowest frequency signal which it is desired to transmit. Accordingly,the

degree'of particularityin its preferred forrn,it is under nd that as ned scl r wn er ed e n has beenmade only by way, of example and thatnumerous changes in the details of construction and the combination andarrangement of parts may be resorted to without de-' ment from :thespirit andscopei of the invention as V 7 hereinafter claimed. 7 a Whatis claimed is:

1- A hase modulator including; a saturablereactor having amagnetic'core, a first winding and a centertapped-second winding ondifferent portions of said core; a phase shifting network including saidfirst winding; and

a modulation circuit including a double. buttoncarbon microphone havingtwo' end terminals and a center-terminal, means for connectingsaidendterminals to up posite 'ends of said second winding, and meansfor connecting said center terminal and thecenter tap of said secondwinding, to -a direct current source, and in which the total resistanceof said modulation circuit ishigh compared with thereactance of saidsecond Winding withing a predetermined band of modulating frequencies 1;2.: A phase modulator including a saturable reactor havingfatle astthree windings/and comprising: a phase shiiting network including :said;first winding and having ,input terminalsf for connection to acarriersource and out-putterminalsifor delivering a phase modulatedoutput; means for connectingsaid,'second windingtogamodnlation sourcehavinga resistance which is high compared withthe reactance of saidsecond winding; means for i n th m i e se dgY -t d i' the m lated outputcomprising a of frectifiei s and means for connecting each or saidrectifiersin series with a direct currentfsourceacross at least a partof said second Pwinding'; and a stabilizing circuit for maintaining the'mean phase angle of said output arounda desired median value, 7 saidstabilizing circuit including a rectifier, the alternating current inputto said circuit being coupled between an output terminal of the phasemodulator and a point or fixedtphase anglewith reference to the carriersource, the output of said circuit being connected to said thirdwinding.

3. The combination of claim 2 including means for adjusting the voltageof said direct current source, for the purpose of adjusting the maximumfrequency deviation.

4. The combination of claim 2 including a permanent magnet for biasing amagnetic core of said saturable reactor;

5. A phase modulator for connection to an alternating current sourcehaving a constant carrier frequency, and

for connection to a modulating source, andincluding a saturable reactorhaving a saturable magnetic core and a carrier frequency winding and amodulating Winding, and

' also including first and second output terminals for delivera ,8havingaresistancewhich is high compared with the reactanceoi saidmodulating winding over a desired .bandof modulating I frequencies;whereby the inductance, of said carrier jrequency winding is varied toproduce avariation inphase'angle of theyoltage across said outputterminals which variation is substantially independent of the modul girq ency a a 1 t 6. The combination of claim 5 including a pair ofrectifier-s connected across said modulating winding, for limiting themaximum phase deviation toa value which is' substantially converselyproportional to the'modulatns' q en Y-H. V 7. The combination of claimSincludinga a permanent magnet for magnetically biassing said core.

8: 'I'hecombination of claim 5 including athird winding on saidsaturable reactor, and a stabilizing circuit a for maintaining the meanphase angle of said output around a desired median value, saidstabilizing circuit including a rectifiegt-he alternating current inputto said circuitbeing coupled between said second output terminal and apoint .offixed. phase angle with reference to said carrier source; theoutput of said stabilizing circuit being connected to said thirdwinding.

9 A phase modulator comprising: a'saturablereactor having a saturablemagnetic coreand a carrier frequency winding and a modulating winding; acondenser connected a series'resonant circuit with saidcarrier frequencyvv-inding with a output terminal therebetween; means for energizing saidseries resonant circuit from a carrier ;frequency input source; and OtheIfClIjCHlt means forestablishinga potentialrat a second output terminal;a modulation circuit including said modulating-winding seriallyconnected to a modulation source having a resist ance which is highcompared with the reactance of said modulating winding over a desiredband of modulation frequencies; whereby the phase angle of thevoltagebetween said output terminals varies byan amountwhich issubstantially proportionalto the'voltage of, but substantiallyindependent of the frequency of said modulation source. i V i 10; Thecombination of claim 9 including a pair of rectifiers connected acrosssaid modulating winding.

'11. The combination of claim 9 including a permanentlmagnet formagnetically biassing said core.

'12. The combination of, claim 9 including a third winding on saidsaturable reactor, and a stabilizing cirquit for maintaining the meanphase angle of said output around a desired median value, saidstabilizing circuit including a rectifienthe alternating current inputto said stabilizing circuit being coupled between said first outputterminal and a point of fixed phase angle with reference to the carriersource; the'output of said stabilizing circuitbeing connected to saidthird winding.

References Cited in the file' of this patent UNITED STATES PATENTS

